This invention relates to an MTI (moving target indication) radar for use in combination with an antenna device at least in detecting a moving target in the presence of Weibull clutter that is known in the art and will later be discussed to some extent. Although not restricted, a radar according to this invention is suited to an air traffic control radar, such as an ASR (airport surveillance radar).
As will later be described in detail with reference to one of several figures of the accompanying drawing, a general radar system that may be an ASR system comprising an antenna, comprises a transmitter for generating a succession of microwave pulses in order to make an antenna radiate the microwave pulses as a sharp directive beam into a space. The antenna is either mechanically or electrically controlled to make the beam repeatedly scan a predetermined solid angle of the space, such as the whole azimuth with a variable elevation angle, along a plurality of unit azimuth regions. The space scanned by the beam will often be called a predetermined space and may be a two-dimensional space with the antenna made to radiate the beam at a predetermined elevation angle.
When an object is present in the scanned space, an echo returns as a return pulse in response to each of a certain number of the radiated microwave pulses either to the antenna or to another similarly controlled antenna for use in receiving such a return signal from each unit azimuth region. In order to facilitate detection of a target, namely, an object to be detected, a receiver output signal produced by a receiver of the radar system is usually used to produce a visual display in which the target is included. The display is used in deciding, with reference to the control of the antenna or antennae, the azimuth of the target or the longitude thereof, as called in terms of spherical polar coordinates, and the elevation angle of the target or the colatitude thereof. In general, the receiver output signal is used also for calculation or measurement of the range or distance of the target from the antenna or antennae with reference to that one of the radiated microwave pulses from which the return pulse is produced. It is possible to use the display in estimating the range.
In practice, it is not seldom that at least one spurious object is present in the scanned space regardless of presence and absence of the target. Besides a target return or echo, namely, the return pulse from the target, such spurious objects produce spurious returns, which result in clutter in the visual display. Although the spurious objects are often referred to as clutter, the word "clutter" will be used in the instant specification primarily to mean the spurious returns irrespective of utilization or not of a visual display. As the case may be, signals produced in the receiver from the target return and the clutter will be called a target return and clutter, respectively.
Examples of the clutter for an MTI radar are land or ground clutter resulting from buildings and undulating terrains, sea clutter arising from sea surface, weather clutter originating with rainfall and rain clouds, and angel echoes attributed to other foreign matters, such as large flocks of migratory birds and/or atmospheric discontinuity or hererogeneity. The difference between the target return and the clutter depends on the field of use of the radar. For instance, the weather clutter becomes the target returns for a weather or meteorological radar. The following description will therefore be limited to MTI radars.
For an MTI radar, it is desirable that the receiver may respond to a moving target with an excellent S/C (signal or target return to clutter) ratio. In other words, the clutter should be suppressed in the receiver output signal to a level of the order of the noise inherent to the receiver. A known MTI (here, a moving target indicator) or MTI canceller is well adapted to discrimination of the moving target from the land clutter but is ineffective in rejecting the clutter caused by spurious objects having velocity components as, for example, the sea clutter, the weather clutter, and the angel echoes. Various proposals have therefore been made to raise the S/C ratio as will presently be described.
By the way, the return signal has an amplitude that varies with time due to target returns and clutter. Furthermore, the clutter is also variable with time. It was formerly believed that the amplitude variation resulting from the clutter follows Rayleigh distribution, which will shortly be described. Later, most of the clutter was found to follow Weibull distribution.
By the use of a variate x representative of the clutter amplitude, which is either zero or positive, the Weibull distribution is expressed by a probability density function (P.sub.W (x) as: EQU P.sub.W (x)=(.eta./.sigma.).multidot.(x/.sigma.).sup..eta.-1 .multidot.exp[-(x/.sigma.).sup..eta. ], (1)
wherein .sigma. and .eta. (sometimes denoted by .gamma.) represent a first or scale and a second or shape parameter, respectively. These parameters have values dependent of the clutter amplitude variation. The Rayleigh distribution is given by another probability density function P.sub.R (x) as: EQU P.sub.R (x)=(2x/.sigma..sup.2).multidot.exp[-(x/.sigma.).sup.2 ], (2)
by the use of the first parameter of the Weibull distribution probability density function (P.sub.W (x) alone. The Rayleigh distribution is therefore the Weibull distribution of a special case where the second parameter behaves as an invariant having a specific value equal to two.
The clutter having an amplitude that follows the Weibull distribution is named Weibull clutter. The clutter having an amplitude that is given by the Rayleigh distribution is called Rayleigh clutter. General guidelines about the Weibull clutter were discussed in detail by D. Curtis Schleher in his article contributed to IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-12, No. 6 (November 1976), pages 736-743, and titled "Radar Detection in Weibull Clutter."
Amongst the proposals for raising the S/C ratio, CFAR (constant false alarm rate) techniques are most promissing. The CFAR techniques are for attaining a constant false alarm rate or probability even in the presence of the clutter. Stated otherwise, the CFAR techniques are to render that false alarm rate constant which represents the probability that the clutter is erroneoulsly detected as a target return.
CFAR processors or detectors for the Rayleigh clutter were reviewed in detail by Vilhelm Gregers Hansen and Harold R. Ward in an article they contributed to IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-8, No. 5 (September 1972), pages 648-652, under the title of "Detection Performance of the Cell Averaging LOG/CFAR Receiver." The term "LOG/CFAR receiver" stands for a logarithmic amplification and CFAR processing receiver. A sophisticated cell averaging LOG/CFAR processor for the Rayleigh clutter of the type reviewed in the Hansen et al article is used in each of MTI radars according to the present invention and will therefore be described later in conjunction with another of the accompanying drawing figures.
A CFAR processor for the Weibull clutter was proposed by Gene B. Goldstein in his article that appeared in IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-9, No. 1 (January 1973), pages 84-92, and was titled "False-Alarm Regulation in Log-Normal and Weibull Clutter." Insofar as the Weibull clutter is concerned, the Goldstein processor is operable only in a specific case for which the parameters for the Weibull distribution are invariants of particular values.
Hansen solely proposed another CFAR processor for the Weibull clutter in general in his report that was made public at International Conference on Radar-Present and Future, 23-25 October 1973. The report of Hansen is paged 1-8 and titled "Constant False Alarm Rate Processing in Search Radars." A processor according to the Hansen report is used in an MTI radar according to an aspect of the instant invention. The processor will therefore be described later with reference to still another of the accompanying drawing figures. Briefly speaking, the Hansen processor carries out suppression of the clutter by converting a variate representative of the clutter amplitude given by the Weibull distribution to a new variate z indicative of the clutter amplitude that follows a simple exponential distribution probability density function P.sub.E (z) given by: EQU P.sub.E (z)=exp(-z). (3)
At any rate, an excellent S/C ratio is achieved by a CFAR processor of the type reported by Hansen. Inasmuch as the Rayleigh clutter is the Weibull clutter of a special case, the processor involves no problem in theory in treating the Rayleigh clutter as the Weibull clutter. The processor is, however, disadvantageous in practice when the clutter merely follows the Rayleigh distribution. This is because the processor must carry out more processes than a sophisticated cell averaging LOG/CFAR processor for the Rayleigh clutter alone. As a result, not only complicated hardware is indispensable but also an increase in error is inevitable to make a clutter residue appear in the receiver output signal when the clutter amplitude is given by the mere Rayleigh distribution.